The present invention relates to a process for preparing zeolite-bound FAU structure type zeolites containing reduced amounts of zeolite P and with good mechanical strength and the use of the zeolite-bound FAU structure type zeolite as prepared by the process as an adsorbent or as a catalyst in hydrocarbon conversion.
Crystalline microporous molecular sieves, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, the crystalline microporous molecular sieves have been used as adsorbents and catalyst carriers for various types of hydrocarbon conversion processes, and other applications. These molecular sieves are ordered, porous, crystalline material having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. The dimensions of these channels or pores are such as to allow for adsorption of molecules with certain dimensions while rejecting those of large dimensions. The interstitial spaces or channels formed by the crystalline network enable molecular sieves such as crystalline silicates, crystalline aluminosilicates crystalline silicoalumino phosphates, and crystalline aluminophosphates, to be used as molecular sieves in separation processes and catalysts and catalyst supports in a wide variety of hydrocarbon conversion processes.
Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable actions such as alkali or alkaline earth metal ions. Although the term xe2x80x9czeolitesxe2x80x9d includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms xe2x80x9czeolitexe2x80x9d, xe2x80x9czeolitesxe2x80x9d and xe2x80x9czeolite materialxe2x80x9d, as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as gallosilicates, silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term xe2x80x9caluminosilicate zeolitexe2x80x9d, as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
Zeolites having a FAU structure type can be employed in adsorption and also as catalysts in the conversion of organic compounds such as hydrocarbons. The details of the structure of these zeolites as well as their isostructures are provided in xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d, eds. W. H. Meier, D. H. Olson and Ch. Baerlocher, Elsevier, Fourth Edition, 1996, which is hereby incorporated by reference. Meier et al. indicates that FAU structure type zeolites are formed by 12-ring structures and have channels of about 7.4 xc3x85. Examples of such zeolites include faujasite, zeolite X, zeolite Y, LZ-210, SAPO-37, zincophosphate X, beryllophosphate X. Other isotopic framework structures include EMT-FAU structure intermediates such as CSZ-1, ECR-30, ZSM-3, and ZSM-20.
Synthetic zeolites, including FAU structure type zeolites, are normally prepared by crystallization of zeolites from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although the zeolite powder has good adsorptive properties, its practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using the powder in commercial processes, the zeolite crystals are usually bound.
The zeolite powder is typically bound by forming a zeolite aggregate such as a pill, sphere, or extrudate. Extruding the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate usually forms the extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. Examples of binder materials include amorphous materials such as alumina, silica, titania, and various types of clays. It is generally necessary that the zeolite be resistant to mechanical attrition, that is, the formation of fines, which are small particles, e.g., particles having a size of less than 20 microns.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when such a bound zeolite is used for hydrocarbon conversion, the performance of the zeolite catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the binder. For instance, since the binder is typically present in an amount of up to about 50 wt. % of zeolite, the binder dilutes the adsorption properties of the zeolite aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite when used in hydrocarbon conversion. Furthermore, when the bound zeolite is used in hydrocarbon conversion, the binder may affect the chemical reactions that are taking place within the zeolite and also may it catalyze undesirable reactions, which can result in the formation of undesirable products.
One procedure for making zeolite-bound zeolites involves converting the silica present of silica-bound aggregates containing zeolite core crystals to a zeolite binder by aging the silica-bound aggregates in an aqueous alkaline solution. The contents of the solution and the temperature at which the aggregates are aged are selected to convert the silica binder to the desired zeolite binder. When such a procedure is used to prepare zeolite-bound FAU structure type zeolite, certain problems can arise. For instance, it is sometimes difficult to convert the silica binder to the desired zeolite binder without also forming unwanted zeolite P. If aging conditions are selected to reduce the formation of zeolite P, the resulting zeolite-bound FAU structure type zeolite may have reduced mechanical strength, which is not desirable if the zeolite-bound FAU structure type zeolite is used in commercial applications.
Thus, the combined objective of producing zeolite-bound FAU structure type zeolite containing reduced amounts of zeolite P and good mechanical strength is somewhat irreconcilable using these procedures.
In accordance with the present invention, there is provided a process for producing zeolite-bound FAU structure type zeolites containing reduced amounts of zeolite P and having good mechanical strength. The process is carried out by converting the silica binder of a silica-bound aggregate containing FAU structure type zeolite in an aqueous mixture comprising an effective amount of crown ether to reduce the formation of unwanted zeolite P and sufficient hydroxy ions to cause the silica binder to be converted to the zeolite binder.
In another embodiment, the present invention provides a hydrocarbon conversion process for converting organic compounds by contacting the organic compounds under hydrocarbon conversion conditions with the zeolite-bound FAU structure type zeolite synthesized by the process. Examples of such processes include reactions such as catalytic cracking, hydrocracking, and reforming. The zeolite-bound FAU structure type zeolite as synthesized by the process can also be employed as adsorbents for performing the selective adsorption of molecules.